1. Field of the Invention
The present invention relates generally to a stack of fuel cells which convert the latent chemical energy of a fuel into electricity directly and, more particularly, is concerned with an improved internal electrolyte supply system for reliable transport of electrolyte throughout the fuel cell stack.
2. Description of the Prior Art
One common fuel cell system includes a plurality of subassemblies which except for the top and bottom subassemblies, each include two bipolar plates between which is supported two gas electrodes, one an anode and the other a cathode, and a matrix with an ion-conductive electrolyte, such as phosphoric acid, between the anode and cathode electrodes. The subassemblies, herein referred to as fuel cells, are oriented one atop another and electrically connected in series (alternate electron and ion paths) to form a fuel cell stack. The top end plate of the top subassembly and the bottom end plate of the bottom subassembly are each half-bipolar plates. Representative examples of such fuel cell system are disclosed in U.S. patents to Kothmann et al (U.S. Pat. Nos. 4,276,355; 4,342,816), Kothmann (U.S. Pat. Nos. 4,292,379; 4,324,844; 4,383,009) and Pollack (U.S. Pat. No. 4,366,211) which, with the exception of U.S. Pat. Nos. 4,342,816 and 4,383,009, are assigned to the assignee of the present invention.
Process gases, such as a fuel and an oxidant are supplied respectively to the anode and cathode electrodes via manifolds attached to the stack and channels defined in the bipolar plates. The fuel in the form of hydrogen atoms when supplied to the anode electrode dissociates into hydrogen ions and electrons. The electrons are transmitted from the anode electrode of a given cell across one bipolar plate to the cathode electrode of an adjacent cell, while the hydrogen ions migrate directly through the acidic electrolyte to the cathode electrode of the given cell, where they react with electrons transmitted to the cathode electrode across the other bipolar plate from the anode electrode of the other adjacent cell and with oxygen to form water. This is repeated at and between the cells throughout the stack with electrons then transferring from the last cathode electrode at one end of the stack to the last anode electrode at the other end of the stack in the form of an electrical current through an external circuit where useful work is produced.
The above-described phosphoric acid fuel cell stack for generating electric power is made up of hundreds of stacked plates, a majority being bipolar plates and a minority being cooling plates, which form a column approximately eight feet in height. Each anode electrode is located on the top side of a bipolar plate facing upward, whereas each cathode electrode is located on the bottom side thereof facing downward. Electrolyte is supplied to the fuel cells in the stack through fill holes at the top of the stack. Most of the plates have electrolyte flow grooves defined on the top surface along a pair of the opposite edges of the plate. These grooves are located below the matrix which is positioned between the electrodes of the cell and distribute the electrolyte across the cell. Also, the plates have vertical holes defined therethrough at selected ends of the grooves such that the grooves and holes form a pair of internal independent serpentine feed paths of electrolyte flow from the top fill holes downward through the fuel cell stack. Similar internal electrolyte supply systems with single serpentine feed path configurations are disclosed in above-cited U.S. Pat. No. 4,383,009 and U.S. Pat. No. 4,572,876 first application cross-referenced above, although in these systems the grooves are located such that the electrolyte flow path is above the matrix.
Problems have been encountered in fuel cell stacks having the above-described construction and electrolyte supply system, the latter being characterized as an internal electrolyte single pass supply system. For reasons not yet fully understood, electrolyte frequently does not feed very far down into the stack before it is stopped. This condition causes the fuel cells at the lower portion of the stack to be dry; without electrolyte the cells do not function as intended. Apparently, the electrolyte fails to move through a groove due to a blockage which results in hydrostatic pressure that damages some fuel cells by flooding and starves the balance of the cells lower down in the stack. Thus, because of the potential for blockage, the internal electrolyte single pass serpentine flow system although relatively simple in construction is unreliable in operation.
One proposed solution to the aforementioned problems is to feed electrolyte to the stack at many elevations from an external system of manifolds. However, this approach presents further problems of attaching the feed and drain lines to the plates and finding the space to put the supply tubes and manifolds.
Consequently, a need exists for an improved internal electrolyte supply system for the fuel cell stack which will ensure reliable flow of electrolyte to all fuel cells of the stack regardless of their particular elevation therein without creating a hydrostatic head in any cell which would cause flooding and electrolyte loss into the process grooves.